Optical connector, particularly for operating in high pressure environment

ABSTRACT

The invention concerns an optical fibre connector ( 15, 18 ) for high pressure (P) environments. Said connector comprises means for maintaining ( 1, 2, 19   b ) the respective ends of the fibres, facing one another. The invention is characterised in that it further comprises a sleeve ( 3 ) enclosing the fibre ends, and a translucent gel ( 4 ), provided in the sleeve to soak the space ( 4   b ) separating the fibre ends ( 15, 18 ). Moreover, by maintaining itself by capillary action in said space ( 4   b ) the gel seals off the connection with environment. The ambient pressure (P) then acts on one free end of the sleeve ( 3 ) while maintaining the gel ( 4 ) in the sleeve.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority of International application numberPCT/FR00/00401, filed Feb. 17, 2000, which in turn claims priority toFrench patent application number 99/02097, filed Feb. 19, 1999.

The present invention relates to the field of optical connections. Mostspecifically, it relates to a connector between two optical fibers,designed to operate in aggressive and/or polluting ambient media, underhigh pressure.

A connection between two optical fibers is made by placing therespective ends of the fibers one facing the other and, preferably,substantially in contact.

However, the surrounding medium may infiltrate between the two fibersthus generating optical losses. A sealed connection is generallydesirable between the two fibers so as to prevent contamination orpollution of their ends.

The connectors normally used for such applications have complexstructures which put a strain on their manufacturing costs (mostcommonly, seals and/or non-return valves). Moreover, this structure isgenerally bulky, making the use of such a connector prohibitive forcertain applications.

The present invention aims to improve the situation.

The invention thus concerns a connector to connect two optical fibersdesigned to engage one with the other, especially in a high-pressuresurrounding medium. This connector comprises means for holding therespective ends of the fibers, substantially one facing the other.

According to a general characteristic of the invention, the connectorcomprises, in addition, a sleeve surrounding the ends of the fibers, anda substantially translucent gel, placed in the sleeve and of fluiditychosen in order to substantially fill the space separating the ends ofthe fibers and to substantially seal this space from the surroundingmedium thereby being held by capillary action. The ambient pressure thenacts on at least one free end of the sleeve while substantially holdingthe gel in the sleeve.

Advantageously, the gel has a refractive index chosen in order to limitthe optical losses from one end of the fiber to the other, in particularFresnel losses.

Preferably, this gel is made from a material comprising silicones.

According to another advantageous characteristic of the invention, thelength and the general diameter of the sleeve are chosen, depending onthe lengths of the ends of the fibers, such that the sleeve forms areservoir for the gel while allowing the gel to fill the interfacebetween the fibers at the time of the connection.

Other advantages and characteristics of the present invention willappear on examining the detailed description below, and the appendeddrawings, in which:

FIG. 1 shows schematically a system to carry out optical measurements ina high-pressure surrounding medium, and comprising an optical connectoraccording to the invention,

FIG. 2 shows a detailed view of the connector according to theinvention, before connecting the aforementioned optical fibers, and

FIG. 3 shows a detailed view of the connector, with the respective endsof the optical fibers connected.

The drawings contain mostly elements of a certain character. They willbe able not only to serve to make the present invention more clearlyunderstood, but also to contribute to its definition, as the case maybe. Reference is now made firstly to FIG. 1 in order to describe asystem for measuring, by optical means, the level of sea-bed salinity inthe example described.

The system comprises an emitting source 10, fitted with an emittingdiode coupled to a first branch of the system 11 comprising an opticalemitting fiber E. Moreover, this emitting fiber is connected to atransmission fiber 18.

The light beam emitted by the source 10 is carried by the transmissionfiber 18 toward a measuring fiber 15 in direct contact with thehigh-pressure surrounding medium (arrows P). In the example described,the tip of the measuring fiber 15 is profiled in order to measure indexvariations of the ambient medium, which makes it possible to detectregions of greater salinity.

In practice, an incident beam is reflected to a greater or lesser degreeby the dioptric interface formed by the measuring fiber 15 and theambient medium, depending on the respective refractive indices of thefiber and of the medium. Hence for a ray with an angle of incidencegreater than the Brewster angle, the incident ray undergoes at least onereflection and is carried back by the transmission fiber 18 up to asplitter 14 (a semireflecting or other plate) which directs thereflected beam to a second receiving branch R which the systemcomprises. This receiving branch R is fitted with a fiber 12, whichfiber is connected to a receiver 13 (photodetector or other detector).

Depending on the light intensity of the reflected beam, it is possibleto calculate the refractive index of the medium surrounding themeasuring fiber 15, and to determine, as required, its composition.

Finally, the system comprises cables (electric cables in the example)connected to the emitting source 10 in order to power the aforementioneddiode, and to the receiver 13 in order to exploit a signalrepresentative the intensity of the reflected beam.

All the elements of the system, other than the downstream part of thedetecting fiber 18 and the measuring fiber 15, are arranged in ahermetic box (isobaric box), which is mainly filled with air of pressureP₀, substantially close to atmospheric pressure. The upstream part ofthe transmission fiber 18 (on the splitter 14 side) fills the hermeticbox 16 whereas its downstream part (on the detecting fiber 15 side) ishoused in a protective sheath 19. The hermeticity of the box 16 isespecially provided by seals 17 surrounding the protective sheath 19.

The transmission fiber, fitted with its sheath, is then designed to beconveyed into the surrounding medium at high pressure P, toward aconnector 20 in order to be connected to the optical measuring fiber 15.

In the intended application, the surrounding medium (sea water)comprises well-known contaminants of normal optical fibers, generallymade from a material comprising silica and chlorides. In this way, theoptical measuring fiber 15, designed to engage directly with the medium,is usually made from a more resistant material, for example sapphire, orelse one comprising a protective diamond layer. However, such materialsare relatively expensive and the measuring fiber 15 has to be of shortlength.

In contrast, the downstream part of the transmission fiber 18 must belong enough, in particular to protect the hermetic box 16 from allimpacts against the bottom (rock, sand). Its protective sheath 19 isthus preferably made from a material resistant to attack by sea water,while the transmission fiber can be made from a conventional material(comprising silica and chlorides).

However, in order to prevent, in particular, contamination of the end ofthe transmission fiber 18 facing the measuring fiber 15, it is necessaryto provide a sealed connector 20 between the two fibers, particularlygiven the high pressure P of the ambient medium (a few hundred bar).

Reference may now be made to FIG. 2 in order to describe a connectoraccording to the invention for connecting the transmission fiber 18 andthe measuring fiber 15.

The protective sheath 19 of the transmission fiber 18 is extended by analignment ferrule 19 b. For the purpose of making the connection betweenthe fibers easier, the measuring fiber 15 is also housed in an alignmentferrule 1, substantially symmetrically with the ferrule 19 b. In theexample, the ferrules are of hollow cylindrical shape in order to housetightly the ends of the fibers 18 and 15. Generally, the fibers areimmobilized in the ferrules by adhesive bonding and their end faces arepolished so as to limit the optical losses in emission and in reception.

The connector comprises a flexible collar 2 of substantially cylindricalhollow shape and of general diameter substantially less than thediameters of the ends of the ferrules 1 and 19 b. The collar 2 isdesigned to keep the respective ends of the ferrules 1 and 19 b, onefacing the other, preferably substantially in contact. The axes of theferrules are then substantially coincident with the X-X alignment axisof the fibers (FIG. 3).

In order to introduce the ferrules 1 and 19 b into the collar 2, thelatter is preferably made from a metal and comprises a slit whichextends along one of its generatrices (parallel to the X-X axis), fromone ferrule to the other. This slit 2 a then gives the collarflexibility by a “spring” effect. The collar 2 is initially fitted overthe end of the ferrule 1 of the detecting fiber 15. Next, the end of theferrule 19 b bearing the transmission fiber 18 is forcedly inserted intothe collar 2, which then closely houses the respective ends of theferrules, one facing the other.

A substantially cylindrical hollow sleeve 3 firstly surrounds the collar2 and the ends of the ferrules 1 and 19 b. In addition, a refractiveindex matching gel 4 is applied inside the sleeve, this forming areservoir for the gel 4.

When the ferrule 19 b of the transmission fiber 18 is introduced intothe sleeve 3, then into the split collar 2, the gel 4, contained in thereservoir formed by the sleeve 3, is distributed by a piston effect inall the interfaces of the connection and in particular in the interface4 b separating the two fibers 15 and 18. The slit 2 a of the collar 2then forms an opening for the gel 4 to flow back, the surplus of whichaccumulates in the cylindrical cavity provided between the sleeve 3 andthe ferrule 19 b (on the transmission fiber 18 side), this cavity againforming a reservoir for the gel 4.

The Applicant has noted that one of the numerous advantages provided bythe present invention thus consists in that this backflow of gel duringconnection drives out any impurities present in the interface 4 b, thesebeing entrained by the gel forced back toward the outside of the collar2. Thus, in addition, connecting fibers by means of a connectoraccording to the invention ensures their ends are cleaned.

Preferably, the collar 2 comprises end faces 2 b substantially flaredoutward, forming a groove to make it easier to introduce the ferrule 19b into the collar.

Moreover, one of the ends of the sleeve 3 (on the detecting fiber 15side, in the example described) is preferably sealed shut on one of itsferrules (ferrule 1).

As shown in FIG. 3, the pressure of the external medium (arrows P) actson the free end of the sleeve 3 (on the transmission fiber 18 side) byholding the gel 4 in the sleeve. In addition, the high-pressure watercan infiltrate via this free end by pushing the gel toward the space 4 bbetween the of the ferrules. The general diameter of the sleeve ischosen, depending on the shape of the ferrules, such that the gel fillsin particular the interface 4 b.

Moreover, the diameter of the sleeve is optimized as a function of thefluidity of the gel 4. In this way, once the fibers are connected, if ithappens that the space 4 b between the fibers increases substantially,part of the gel 4 coming in particular from the cylindrical cavityforming a reservoir occupies the whole interface 4 b by capillary actionin addition.

Preferably, the sleeve 3 is made from a material comprisingfluoropolymers, of the heat-shrinkable KYNAR (registered trademark)type, and one of its ends is closed off by heat-shrinking onto theferrule 1 of the detecting fiber 15.

Thus, the surrounding high-pressure medium contributes to sealing theconnection between the fibers by pushing the gel back toward the insideof the collar, in particular between the two ferrules. The gel, itselfsubjected to the external pressure P, does not leak.

Advantageously, this gel, made from a material comprising silicones, isresistant to sea water (not washed out), does not coagulate and hardlyexpands or contracts with any variations in temperature or pressure.Thus, the only dioptric interfaces formed between the two fibers are theinterfaces between the gel and the respective ends of the fibers.Moreover, the refractive index of the gel is chosen so as to limit theoptical losses linked to the connection (Fresnel losses). This isbecause the rays passing through the gel remain concentrated on the X-Xaxis.

According to another advantage provided by the present invention, theconnector thus formed has a compact overall structure and, inparticular, quite a thin profile in order not to generate perturbationsin the measurements, which would result from a variation in the flow ofthe surrounding medium, caused by the connector.

Moreover, the simplicity of assembling the connector in the exampledescribed above makes it possible to quickly replace the measuring probeof the system (measuring fiber 15, alignment ferrule 1, collar 2 andsleeve 3), if the detecting fiber 15, made from sapphire in the exampledescribed, breaks following a shock.

Of course, the present invention is not limited to the embodimentdescribed above by way of example. It extends to other variants.

Thus it will be understood that the measuring fiber 15 may carry outmeasurements other than those of refractive index, for examplefluorescence measurements, ellipsometry measurements or othermeasurements. For example, for fluorescence measurement, the splitter 14in FIG. 1 may comprise an optical interference filter, preferablycentered on the emission wavelength of the aforementioned diode. Such afilter mainly transmits the wavelength of the diode and substantiallyreflects the fluorescence wavelength. Advantageously, the gel 4maintains the polarizations of the emitted and received beams. Thus, inaddition, it is possible to carry out the aforementioned ellipsometrymeasurements by modulating in particular the polarization of the emittedbeam.

Moreover, the means for holding the fibers one facing the other (splitcollar 2 and alignment ferrules 1 and 19 b) are described above by wayof example. Any other type of means for holding fibers can beenvisioned, provided it is not fully sealed (such as split collar 2 inthe example), in order to allow the surrounding medium to exert pressureon the ends of the sleeve and on the gel, in particular by allowing thegel to fill the interface between the two ends of the fibers 15 and 18.

The materials in which the elements of the connector 20 are made arementioned above by way of example. However, the gel, made from amaterial comprising silicones, has the advantages described above (noteasily washed out by sea water, not easily coagulated, adequatefluidity). Moreover, the sleeve made from KYNAR and the nonoxidizingalignment ferrules are generally little affected by chemical attack bysea water.

Of course, the cylindrical shapes of the various elements of theconnector are described above by way of example, and are likely toexhibit variants.

In addition, the application of the connector according to the inventionto the measurement of the level of salinity of seabeds from therefractive index of sea water is described above by way of example. Theinvention also finds applications in other fields, such asagri-foodstuffs (for example in measuring the butter content in milk),or in studying the composition of heterogeneous media (comprising forexample gases, water and/or oils, of different respective refractiveindices)

In the application described above by way of example, one of theconnected fibers 15 comprises, at some distance from the connector, anend comprising an optical sensor for measuring the variations inrefractive index. As a variant, the connector according to the inventioncan also be applied to the connection between two transmission fibers,for example.

Finally, the present invention can in addition relate to a method ofconnection with a connector according to the invention, in particularcomprising the step of introducing the ferrule 19 b into the sleeve 3comprising the gel 4 (FIGS. 2 and 3).

What is claimed is:
 1. A connector for optical fibers for use in highpressure atmospheres having means for holding respective ends of firstand second optical fibers such that the respective end of the firstfiber substantially faces the respective end of the second fiber, theholding means comprising: a first cylindrical hollow alignment ferrulefor housing and immobilizing the respective end of the first fiber; asecond cylindrical hollow alignment ferrule for housing and immobilizingthe respective end of the second fiber; a substantially flexible, hollowcollar having a substantially cylindrical shape, wherein diameter of theflexible collar is less than diameter of the first and second alignmentferrules to closely house respective ends of the first and secondalignment ferrules substantially one facing the other; a sleevesurrounding the collar; and a substantially translucent gel placed inthe sleeve, the fluidity of the translucent gel adapted to substantiallyfill a space separating the respective ends of the first and secondfibers and to substantially seal the space from a surrounding mediumthereby holding the respective ends of the first and second fibers inthe space by capillary action, whereas ambient pressure in thesurrounding medium acts on at least one free end of the sleeve whilesubstantially holding the gel in the sleeve; wherein the collarcomprises a slit forming a passage for the gel extending substantiallyfrom the first ferrule to the second ferrule.
 2. The connector asclaimed in claim 1 wherein the gel has a refractive index chosen inorder to limit the optical losses from one end of the fiber to theother.
 3. The connector as claimed in claim 2, wherein the gel is madefrom a material comprising silicones.
 4. The connector as claimed inclaim 1 wherein the general diameter of the sleeve is chosen such thatthe gel is able to be kept at least between the respective ends of thefirst and second fibers by capillary action, and in a cavity whichcommunicates with the surrounding medium and is formed between aninternal surface of the sleeve and the first and second ferrules facingthe free end of the sleeve.
 5. The connector as claimed in claim 1wherein the sleeve comprises a free end, communicating with thesurrounding medium, and an end which is substantially closed around oneof the first or second fibers.
 6. The connector as claimed in claim 1further comprising an optical sensor coupled to an end opposite of therespective end of either the first or second fiber.
 7. The connector asclaimed in claim 1 wherein the diameter of at least a portion of theflexible collar expands in order to closely house respective ends of thefirst and second alignment ferrules substantially one facing the other.